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Underwood (1957) provided early evidence that things you've learned before encoding a target item can worsen recall of that target item. In a meta-analysis of multiple experiments, he showed that the more lists one had already learned, the more trouble one had in recalling the most recent one. This is proactive interference, where the prior existence of old memories makes it harder to recall newer memories.

Proactive interference can be potently demonstrated with the Brown-Peterson paradigm (Brown, 1958; Peterson & Peterson, 1958). A single Brown-Peterson trial consists of a study list, a retention interval and then a recall period. The study list might consist of a handful of related items (such as a handful of animals or occupations), presented individually every few seconds. For the duration of a short retention interval, subjects are then asked to perform an engaging distractor task such as counting backwards in sevens (to minimize rehearsal). Finally, subjects are asked to recall the items from this study list.

Usually, subjects' back side recollection is nearly perfect for the first trial, but perform increasingly poorly on subsequent trials that use study lists drawn from the same category. This is the proactive interference effect described earlier. In other words, even though the lists from previous trials are now irrelevant, the fact that they were studied at all is somehow making it harder for subjects
to recall the most recent list.

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Proactive interference occurs with memories being learned in similar contexts. It is also associated with poorer list discrimination, which occurs when participants are asked to judge whether an item has appeared on a previously learned list.[1] Moreover, if the items or pairs to be learned are conceptually related to one another, then proactive interference has a greater effect.[2]

The leading experimental technique for studying proactive interference in the brain is the “recent-probes” task, in which participants must commit a given set of items to memory and they are asked to recall a specific item indicated by a probe.[3] Using the recent-probes task and fMRIs, the brain mechanisms involved in the resolution of proactive interference have been identified as the ventrolateralprefrontal cortex and the left anterior prefrontal cortex.[4]

Researchers have studied the joint influence of proactive and retroactive interference using a list of items to be remembered. As expected, recall was hampered by increasing the number items in a given list.[5] Proactive interference also affected learning when dealing with multiple lists. Researchers had participants learn a list of 10 paired adjectives.[6] The experimenters would consider a list to be learned if the participant could correctly recall eight of the ten items. After two days, participants could recall close to 70% of the items. However, those asked to memorize a new list the day after learning the first one had a recall of only 40%. Those who learned a third list recalled 25% of the items. Therefore, Proactive interference affected the correct recall of the last list learned, because of the previous one, or two. In terms of forgetting, the effect of Proactive interference was proven by further studies using different methods.[7] The effect of proactive interference was reduced when the test was immediate and when the new target list was obviously different from the previously learned lists.

Span performance refers to working memory capacity. It is hypothesized that span performance is limited in language comprehension, problem solving, and memory.[8] Proactive Interference affects susceptibility to span performance limitations, as span performance in later experimental trials were worse than performance in earlier trials.[8][9] With single tasks, proactive interference had less effect on participants with high working memory spans than those with low ones. With dual tasks, both types were similarly susceptible.

To differ, others have tried to investigate the relation of proactive interference when cued to forget. Turvey and Wittlinger designed an experiment to examine the effects of cues such as "not to remember" and "not to recall" with currently learned material. While "not to remember" had a significant effect in reducing proactive interference, cued to "not to recall" previously encoded and stored information did not significantly reduce the effect. Therefore, these associated cues do not directly control the potential effect of proactive interference on short term memory span.[10]

Proactive interference has shown an effect during the learning phase in terms of stimuli at the acquisition and retrieval stages with behavioral tasks for humans, as found by Castro, Ortega and Matute.[11] With 106 participants, they investigated two main questions: if two cues are learned as predictors of the same outcome (one after the other), would the second-cue outcome association be retarded? And secondly, once the second association is fully learned, will there still be an effect on subsequent trials? The research, as predicted, showed retardation and impairment in associations, due to the effect of Proactive Interference.